Chirped coherent laser radar system and method

ABSTRACT

A laser radar system using collocated laser beams to unambiguously detects a range of a target and a range rate at which the target is moving relative to the laser radar system. Another aspect of various embodiments of the invention may relate to a laser radar system that uses multiple laser radar sections to obtain multiple simultaneous measurements (or substantially so), whereby both range and range rate can be determined without various temporal effects introduced by systems employing single laser sections taking sequential measurements. In addition, other aspects of various embodiments of the invention may enable faster determination of the range and rate of the target, a more accurate determination of the range and rate of the target, and/or may provide other advantages.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/651,989, filed Feb. 14, 2005, and entitled“Chirped Coherent Laser Radar System and Method.”

FIELD OF THE INVENTION

The invention relates generally to a measuring device and moreparticularly to a chirped coherent laser radar system.

BACKGROUND OF THE INVENTION

Various measuring devices for measuring linear distances using one ormore laser radars are known. Such measuring devices may generateinformation related to a distance or range of a target from themeasuring device and/or a velocity, or range rate, of the targetrelative to the measuring device. This range and range rate informationmay be useful in a variety of settings. For the purposes of thisapplication the term range rate refers to the rate of change in therange between the target and the measuring device.

A typical measuring device may include, for example, a frequencymodulated laser radar system. The system may include a laser source thatemits a beam of electromagnetic radiation. The beam may be emitted at afrequency that is continuously varied, or chirped. In some instances,chirping the frequency may include sweeping the frequency between alower frequency and an upper frequency (or vice versa) in a periodicmanner (e.g. a sawtooth waveform, a triangle waveform, etc.). The beammay be divided into a target beam and a reference beam.

In conventional embodiments, the system may include a targetinterferometer and a reference interferometer. The target interferometermay receive the target beam, and may generate a target signalcorresponding to a frequency difference between one portion of thetarget beam directed towards, and reflected from, the target, andanother portion of the target beam that directed over a path with aknown or otherwise fixed path length. The frequency difference maydetermined by the target interferometer based on an interference signalderived from the two portions of the target beam. The referenceinterferometer may receive the reference beam and may generate areference signal corresponding to a frequency difference between twoportions of the reference beam that may be directed over two separatefixed paths with a known path length difference. The frequencydifference may be determined by the reference interferometer based on aninterference signal derived from the two portions of the reference beam.

Generally, the system may include a processor. The processor may receivethe target signal and the reference signal and may process these signalsto determine the range between the target interferometer and the target.Range information determined based on the target signal and thereference signal may be used to determine a range rate of the targetwith respect to the target interferometer.

Conventional systems may be built, for example, as described in U.S.Pat. No. 5,114,226, entitled “3-DIMENSIONAL VISION SYSTEM UTILIZINGCOHERENT OPTICAL DETECTION,” which is incorporated herein by referencein its entirety.

Conventional systems are typically limited in various aspects ofoperation. For example, these conventional systems are not able toprovide range and/or range rate information instantaneously based on thetarget signal and reference signal, or unambiguously determine distanceand velocity. These conventional systems are limited in other ways aswell. These limitations may be exacerbated by various operatingconditions such as, for example, target acceleration toward or away fromthe target interferometer, using an actuated optical element (e.g. amirror or lens) to scan the target at high speeds, or other operatingconditions.

In some configurations, beams produced by two laser sources may becombined to provide a beam of electromagnetic radiation that may then bedivided into a reference beam and a target beam. In theseconfigurations, the frequencies of the two laser sources may be counterchirped, or, in other words, the two frequencies may be chirped suchthat while a frequency of one of the laser sources is ascending towardan upper frequency, the other is descending toward a lower frequency,and vice versa. Systems utilizing such a configuration may suffer someor all of the drawbacks associated with single laser source systems, aswell as other drawbacks unique to two laser source systems.

SUMMARY

One aspect of various embodiments of the invention may relate to a laserradar system that unambiguously detects a range of a target and a rangerate at which the target is moving relative to the laser radar system.Another aspect of various embodiments of the invention may relate to alaser radar system that uses multiple laser radar sections to obtainmultiple simultaneous measurements (or substantially so), whereby bothrange and range rate can be determined without various temporal effectsintroduced by systems employing single laser sections taking sequentialmeasurements. In addition, other aspects of various embodiments of theinvention may enable faster determination of the range and rate of thetarget, a more accurate determination of the range and rate of thetarget, and/or may provide other advantages.

In some embodiments of the invention, the laser radar system may emit afirst target beam and a second target beam toward a target. The firsttarget beam and the second target beam may be reflected by the targetback toward the laser radar system. The laser radar system may receivethe reflected first target beam and second target beam, and maydetermine at least one of a range of the target from the laser radarsystem, and a range rate of the target. In some embodiments of theinvention, the laser radar system may include a first laser radarsection, a second laser radar section, and a processor.

In some embodiments of the invention, the first laser radar section maygenerate a first target beam and a first reference beam. The firsttarget beam and the first reference beam may be generated by a firstlaser source at a first frequency that may be modulated at a first chirprate. The first target beam may be directed toward a measurement pointon the target. The first laser radar section may combine one portion ofthe first target beam directed towards, and reflected from, the targetwith another portion of the first target beam, referred to as a localoscillator beam, directed over a path with a known or otherwise fixedpath length. This may result in a combined first target beam.

According to various embodiments of the invention, the second laserradar section may be collocated and fixed with respect to the firstlaser radar section. More particularly, the relevant optical componentsfor transmitting and receiving the respective laser beams are collocatedand fixed. The second laser radar section may generate a second targetbeam and a second reference beam. The second target beam and the secondreference beam may be generated by a second laser source at a secondfrequency that may be modulated at a second chirp rate. The second chirprate may be different from the first chirp rate. This may facilitate oneor more aspects of downstream processing, such as, signaldiscrimination, or other aspects of downstream processing. The secondtarget beam may be directed toward the same measurement point on thetarget as the first target beam. The second laser radar section maycombine one portion of the second target beam directed towards, andreflected from, the target, and another portion of the second targetbeam that directed over a path with a known or otherwise fixed pathlength. This results in a combined second target beam.

According to various embodiments of the invention, the processorreceives the first and second combined target beams and measures a beatfrequency caused by a difference in path length between each of therespective reflected target beams and its corresponding local oscillatorbeam (e.g., the first and second combined target beams), and by anyDoppler frequency shift created by target motion relative to the laserradar system. The beat frequencies may then be combined linearly togenerate unambiguous determinations of the range and the range rate ofthe target, so long as the beat frequencies between each of therespective local oscillator beams and the its reflected target beamcorrespond to simultaneous (or substantially simultaneous) temporalcomponents of the reflected target beams. Simultaneous (or substantiallysimultaneous) temporal components of the reflected target beams mayinclude temporal components of the target beams that: 1) have beenincident on substantially the same portion of the target, 2) have beenimpacted by similar transmission effects, 3) have been directed by ascanning optical element under substantially the same conditions, and/or4) share other similarities. The utilization of beat frequencies thatcorrespond to simultaneous (or substantially simultaneous) temporalcomponents of the reflected target beams for linear combination mayeffectively cancel any noise introduced into the data by environmentalor other effects (see e.g. Equation (1)).

Since the combined target beams may be created by separately combiningthe first local oscillator beam and the second local oscillator beamwith different target beams, or different portions of the same targetbeam, the first combined target beam and the second combined target beammay represent optical signals that would be present in two separate, butcoincident, single source frequency modulated laser radar systems, justprior to final processing. For example, the combined target beams mayrepresent optical signals produced by target interferometers in singlesource systems.

According to various embodiments, the target beams may be directed toand/or received from the target on separate optical paths. In someembodiments, these optical paths may be similar but distinct. In otherembodiments the first target beam and the second target beam may becoupled prior to emission to create a combined target beam directedtoward the target along a common optical path. In some embodiments, thecombined target beam may be reflected by the target and may be receivedby the laser radar system along a reception optical path separate fromthe common optical path that directed the target beam toward the target.Such embodiments may be labeled “bistatic.” Or, the combined target beammay be received by the laser radar system along the common optical path.These latter embodiments may be labeled “monostatic.” Monostaticembodiments may provide advantages over their bistatic counterparts whenoperating with reciprocal optics. More particularly, monostaticembodiments of the invention may be less affected by differentialDoppler effects and distortion due to speckle, among other things.Differential Doppler effects are created, for example, by a scanningmirror that directs the target beam to different locations on a target.Since different parts of the mirror are moving at different velocities,different parts of the target beam experience different Doppler shifts,which may introduce errors into the range and or range ratemeasurements. These effects have been investigated and analyzed byAnthony Slotwinski and others, for example, in NASA Langley Contract No.NAS 1-18890 (May 1991) Phase II Final Report, Appendix K, submitted byDigital Signal Corporation, 8003 Forbes Place, Springfield, Va. 22151,which is incorporated herein by reference in its entirety.

In some instances, the first laser source and the second laser sourcemay generate electromagnetic radiation at a first carrier frequency anda second carrier frequency, respectively. The first carrier frequencymay be substantially the same as the second carrier frequency. This mayprovide various enhancements to the laser radar system, such as, forexample, minimizing distortion due to speckle, or other enhancements.

In some embodiments, the first laser source and the second laser sourcemay rely on, or employ, highly linearized components to generate theirrespective laser beams. To this end, the first laser source and thesecond laser source may be linearized on a frequent basis (e.g. eachchirp), or in some embodiments continuously (or substantially so). Thislinearization may provide enhanced range measurement accuracy, or otherenhancements, over conventional systems in which linearization may occurat startup, when an operator notices degraded system performance, whenthe operator is prompted to initiate linearization based on a potentialfor degraded performance, or when one or more system parameters fall outof tolerance, etc. Frequent and/or automated linearization may reducemirror differential Doppler noise effects during high speed scanning andmay maximize the effectiveness of dual chirp techniques for cancelingout these and other noise contributions to range estimates.

In some embodiments of the invention, the laser radar system maydetermine the range and the range rate of the target with an increasedaccuracy when the range of the target from the laser radar system fallswithin a set of ranges between a minimum range and a maximum range. Whenthe range of the target does not fall within the set of ranges, theaccuracy of the laser radar system may be degraded. This degradation maybe a result of the coherence length(s) of the first laser source and thesecond laser source, which is finite in nature. For example, thedistance between the minimum range and the maximum range may be afunction of the coherence length. The longer the coherence length of thefirst laser source and the second laser source, the greater the distancebetween the minimum range and the maximum range. Thus, increasing thecoherence length of the first laser source and the second laser sourcemay enhance range and range rate determinations by the laser radarsystem by providing the ability to make determinations over an enhancedset of ranges.

Accordingly, in some embodiments of the invention, the first lasersource and the second laser source may emit electromagnetic radiationwith an enhanced coherence length. For example, the first laser sourceand/or the second laser source may include a ring cavity laser system.The ring cavity laser system may provide electromagnetic radiation withone or more enhancements such as, for example, an increased coherencelength, a more precise frequency control, a more precise chirp ratecontrol, a more linear chirp, a relatively simple and/or compact opticalconfiguration or other enhancements.

In some embodiments, the ring cavity system may include one or moreoptical elements that may form an optical cavity through whichelectromagnetic radiation may be circulated, an optical amplifier, and afrequency shifting device that may apply a frequency shift to theelectromagnetic radiation circulating through the optical cavity. Thefrequency shifting device may include an acousto-optical modulator, amoving surface or other device. The acousto-optical modulator may applya constant frequency shift to the electromagnetic radiation circulatingthrough the optical cavity which may provide the electromagneticradiation output by the ring cavity system with a substantially linearchirp. The acousto-optical modulator may include an acousto-optic Braggcell. The ring cavity system may form a laser whose natural lasing modeproduces electromagnetic radiation with a linearly varying opticalfrequency and enhanced coherence length.

In some embodiments of the invention, one of the chirp rates may be setequal to zero. In other words, one of the laser sources may emitradiation at a constant frequency. This may enable the laser sourceemitting at a constant frequency to be implemented with a simplerdesign, a small footprint, a lighter weight, a decreased cost, or otherenhancements that may provide advantages to the overall system. In theseembodiments, the laser radar section with chirp rate set equal to zeromay be used to determine only the range rate of the target.

In some embodiments of the invention, the processor may linearly combinethe first combined target beam and the second combined target beamdigitally to generate the range signal and the range rate signal. Forexample, the processor may include a first detector and a seconddetector. The first detector may receive the first combined target beamand may generate a first analog signal that corresponds to the firstcombined target beam. The first analog signal may be converted to afirst digital signal by a first converter. The processor may include afirst frequency data module that may determine a first set of frequencydata that corresponds to one or more frequency components of the firstdigital signal.

The second detector may receive the second combined target beam and maygenerate a second analog signal that corresponds to the second combinedtarget beam. The second analog signal may be converted to a seconddigital signal by a second converter. The processor may include a secondfrequency data module that may determine a second set of frequency datathat corresponds to one or more of frequency components of the seconddigital signal.

The first set of frequency data and the second set of frequency data maybe received by a frequency data combination module. The frequency datacombination module may generate a range rate signal and a range signalderived from the first set of frequency data and the second set offrequency data.

In other embodiments of the invention, the processor may mix the firstcombined target beam and the second combined target beam electronicallyto generate the range signal and the range rate signal. For example, theprocessor may include a modulator. The modulator may multiply the firstanalog signal generated by the first detector and the second analogsignal generated by the second detector to create a combined analogsignal. In such embodiments, the processor may include a first filterand a second filter that receive the combined analog signal. The firstfilter may filter the combined analog signal to generate a firstfiltered signal. The first filtered signal may be converted by a firstconverter to generate a range rate signal. The second filter may filterthe combined analog signal to generate a second filtered signal. Thesecond filtered signal may be converted by a second converter togenerate a range signal.

According to other embodiments of the invention, the processor may mixthe first combined target beam and the second combined target beamoptically to generate the range signal and the range rate signal. Forexample, the processor may include a detector that receives the firstcombined target beam and the second combined target beam and generates acombined analog signal based on the detection of the first combinedtarget beam and the second combined target beam. In such embodiments,the processor may include a first filter and a second filter thatreceive the combined analog signal. The first filter may filter thecombined analog signal to generate a first filtered signal. The firstfiltered signal may be converted by a first converter to generate arange rate signal. The second filter may filter the combined analogsignal to generate a second filtered signal. The second filtered signalmay be converted by a second converter to generate a range signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional laser radar system.

FIG. 2 illustrates a laser radar system according to one or moreembodiments of the invention.

FIG. 3 illustrates a ring cavity system according to one or moreembodiments of the invention.

FIG. 4 illustrates a processor that digitally mixes two combined targetbeams according to one or more embodiments of the invention.

FIG. 5 illustrates a processor that electrically mixes two combinedtarget beams according to one or more embodiments of the invention.

FIG. 6 illustrates a processor that optically mixes two combined targetbeams according to one or more embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional frequency modulated laser radar system110. System 110 typically includes a laser source 112 that emits a beam114 of electromagnetic radiation. Beam 114 may be emitted at a frequencythat is continuously varied, or chirped. In some instances, chirping thefrequency may include sweeping the frequency between a lower frequencyand an upper frequency (or vice versa) in a periodic manner (e.g. asawtooth waveform, a triangle waveform, etc.). Beam 114 may be dividedby an optical coupler 116 into a target beam 118 and a reference beam120.

In conventional embodiments, system 110 may include a targetinterferometer 122 and a reference interferometer 124. Targetinterferometer 122 may receive target beam 118, and may divide thetarget beam at an optical coupler 126. Target interferometer 122 istypically used to generate a target signal that may depend upon a rangeof a target 130 from target interferometer 122. Target interferometermay accomplish this by directing one portion 128 of target beam 118toward target 130, and the other portion 132 of target beam 118 to atarget frequency difference module 134 over an optical path with a fixedpath length. Portion 128 of target beam 118 may be reflected by target130 and may be transmitted to target frequency difference module 134 viaoptical coupler 126 and an optical fiber 136. Portions 128 and 132 maythen be combined at an optical coupler 148. Based on interferencebetween portions 128 and 132 after combination at coupler 148, targetfrequency difference module 134 may generate the target signalcorresponding to a beat frequency of portions 128 and 132 of target beam118 due to the difference between their path lengths.

According to various embodiments of the invention, referenceinterferometer 124 may receive reference beam 120 and may generate areference signal corresponding to a frequency difference between twoportions of reference beam 124 that may be directed over two separatefixed paths with a known path length difference. More particularly,reference beam 120 may be divided by an optical coupler 140 into a firstportion 142 and a second portion 144 and recombined at an opticalcoupler 146. First portion 142 may have a fixed optical path lengthdifference relative to second portion 144. Based on interference betweenportions 142 and 144 after combination at coupler 146, referencefrequency difference module 150 may generate the reference signalcorresponding to a beat frequency of portions 142 and 144 of referencebeam 120 caused by the fixed difference between their path lengths.

As will be appreciated, target interferometer 122 and referenceinterferometer 124 have been illustrated and described as Mach-Zehnderinterferometers. However other interferometer configurations may beutilized. For example, target interferometer 122 and referenceinterferometer 124 may include embodiments wherein Michaelson-Morleyinterferometers may be formed.

In some embodiments, system 110 may include a processor 138. Processor138 may receive the target signal and the reference signal and mayprocess these signals to determine the range of target 130. Rangeinformation determined based on the target signal and the referencesignal may be used to determine a range rate of target 130 with respectto target interferometer 122.

FIG. 2 illustrates an exemplary embodiment of a laser radar system 210that employs two or more laser radar sections, each of which emits atarget beam toward a target. For example, a first laser radar section274 emits a first target beam 212 and a second laser radar section 276emits a second target beam 214 toward a target 216. In some embodimentsof the invention, first target beam 212 and second target beam 214 maybe chirped to create a dual chirp system. According to variousembodiments of the invention, laser section 274 may include a lasersource controller 236, a first laser source 218, a first optical coupler222, a first beam delay 244, a first local oscillator optical coupler230, and/or other components. Second laser radar section 276 may includea laser source controller 238, a second laser source 220, a secondoptical coupler 224, a second beam delay 250, a second local oscillatoroptical coupler 232 and/or other components. For example, some or all ofthe components of each of laser radar sections 274 and 276 may beobtained as a coherent laser radar system from MetricVision™. Coherentlaser radar systems from MetricVision™ may provide various advantages,such as enhanced linearity functionality, enhanced phase wanderingcorrection, and other advantages to laser radar system 210 indetermining the range and the range rate of target 216.

In some embodiments of the invention, first target beam 212 and secondtarget beam 214 may be reflected by target 216 back toward laser radarsystem 210. Laser radar system 210 may receive first target beam 212 andsecond target beam 214, and may determine at least one of a range oftarget 216 from laser radar system 210, and a range rate of target 216.

According to various embodiments of the invention, first laser source218 may have a first carrier frequency. First laser source 218 may emita first laser beam 240 at a first frequency. The first frequency may bemodulated at a first chirp rate. The first frequency may be modulatedelectrically, mechanically, acousto-optically, or otherwise modulated aswould be apparent. First laser beam 240 may be divided by first opticalcoupler 222 into first target beam 212 and a first local oscillator beam242. First local oscillator beam 242 may be held for a first delayperiod at a first beam delay 244.

In some embodiments of the invention, second laser source 220 may emit asecond laser beam 246 at a second frequency. The second frequency may bemodulated at a second chirp rate different from the first chirp rate.The second frequency may be modulated electrically, mechanically,acousto-optically, or otherwise modulated. The first chirp rate and thesecond chirp rate may create a counter chirp between first laser beam240 and second laser beam 246.

In some instances, the second carrier frequency may be substantially thesame as the first carrier frequency. For example, in some embodimentsthe percentage difference between the first baseline frequency and thesecond baseline frequency is less than 0.05%. This may provide variousenhancements to laser system 210, such as, for example, minimizingdistortion due to speckle, or other enhancements. Second laser beam 246may be divided by second optical coupler 224 into a second target beam214 and a second local oscillator beam 248. Second local oscillator beam248 may be held for a second delay period at a second beam delay 250.The second delay period may be different than the first delay period.

In some embodiments, the output(s) of first laser source 218 and/orsecond laser source 220 (e.g. first laser beam 240 and/or second laserbeam 246) may be linearized using mechanisms provided in, for example,METRICVISION™ Model MV200. Phase wandering of the output(s) of firstlaser source 218 and/or second laser source 220 may corrected usingmechanisms provided in, for instance, METRICVISION™ Model MV200.

In some embodiments of the invention, laser radar system 210 maydetermine the range and the range rate of target 216 with an increasedaccuracy when the range of target 216 from laser radar system 210 fallswithin a set of ranges between a minimum range and a maximum range. Whenthe range of target 216 does not fall within the set of ranges, theaccuracy of laser radar system 210 may be degraded.

According to various embodiments of the invention, first beam delay 244and second beam delay 250 may be adjustable. Adjusting first beam delay244 and second beam delay 250 may enable laser radar system 210 to beadjusted to bring the set of ranges over which more accuratedeterminations may be made closer to, or further away from, laser radarsystem 210. First beam delay 244 and the second beam delay 250 may beadjusted to ensure that the range of target 216 falls within the set ofranges between the minimum range and the maximum range so that the rangeand the range rate of target 216 may be determined accurately. Firstbeam delay 244 and second beam delay 250 may be adjusted by a user, orin an automated manner.

The degradation of determinations of range and range rate when the rangeof target 216 is outside of the set of ranges may be a result of thefinite nature of the coherence length of first laser source 218 andsecond laser source 220. For example, the distance between the minimumrange and the maximum range may be a function of the coherence length.The longer the coherence length of first laser source 218 and secondlaser source 220, the greater the distance between the minimum range andthe maximum range may be. Thus, increasing the coherence length of firstlaser source 218 and second laser source 220 may enhance range and rangerate determinations by laser radar system 210 by providing the abilityto make determinations over an enhanced set of ranges.

In some embodiments of the invention, first local oscillator beam 242may be divided into a plurality of first local oscillator beams andsecond local oscillator beam 248 may be divided into a plurality ofsecond local oscillator beams. In such instances, laser radar system 210may include a plurality of beam delays that may apply delays of varyingdelay periods to the plurality of first local oscillator beams and theplurality of second local oscillator beams. This may ensure that one ofthe plurality of first local oscillator beams and one of the pluralityof second local oscillator beams may have been delayed for delay periodsthat may enable the range and range rate of the target to determinedaccurately.

Accordingly, in some embodiments of the invention, first laser source218 and second laser source 220 may emit chirped electromagneticradiation with an enhanced coherence length. Examples of such lasersources are described in U.S. Pat. No. 4,586,184 entitled “ACOUSTICALLYCONTROLLED FREQUENCY SHIFTED CAVITY FOR ELECTROMAGNETIC RADIATION,”which is incorporated herein by reference in its entirety. For example,first laser source 218 and/or second laser source 220 may include a ringcavity system. FIG. 3 illustrates an exemplary embodiment of a ringcavity system 310. Ring cavity system 310 may provide electromagneticradiation with various enhancements such as, for example, an increasedcoherence length, a more precise frequency control, a more precise chirprate control, or other enhancements. Ring cavity system 310 may includea ring fiber 316, an frequency shifting device 318, one or more opticalamplifiers 320, an output fiber 322, and/or other components.

According to some embodiments, the ring cavity system 310 forms a laseroscillator with the optical amplifier(s) 320 providing sufficient gainto spontaneously generate lasing output in fiber 322. Because theoscillator may not be modulated in the conventional sense, theoscillator may be constructed to include an enhanced instantaneousoptical bandwidth operation. For example, the instantaneous opticalbandwidth may be relatively narrow, which may produce electromagneticradiation with a relatively long coherence length. Ring cavity system310 may include frequency shifting device 318 within the optical cavityformed by ring fiber 316. Frequency shifting device 318 may shift thefrequency of the electromagnetic radiation as the electromagneticradiation passes through the optical cavity.

In some instances, frequency shifting device 318 may includeacousto-optic modulator, such as an acousto-optic Bragg cell, or otheracousto-optic modulator, or other frequency shifting devices. Thefrequency shift applied to the electromagnetic radiation applied byfrequency shifting device 318 may be adjusted by an RF source 324. Insome cases, acousto-optic Bragg cell 318 may provide a constantfrequency shift to the electromagnetic radiation to produce asubstantially linear chirp. Output radiation may be output from ringcavity system 310 via output fiber 322. It may be appreciated thatalthough the ring cavity system has been described generally as beingimplemented using optical fibers, that the ring cavity system couldalternatively be implemented using other optical elements (e.g. mirrors,lenses, alternative amplifier elements, etc.).

Ring cavity system 310 may additionally incorporate further enhancements(not shown) such as a switch within the cavity that allows theelectromagnetic radiation within the cavity to be dumped from the ringand/or external electromagnetic radiation to be injected into thecavity. Such enhancements may, for example, allow for enhanced controlover the operation of the laser oscillator by stopping and/or startinglasing at particular wavelengths or at particular times.

According to various embodiments, first target beam 212 and secondtarget beam 214 may be directed and/or received from target 216 onseparate optical paths. In some embodiments, these optical paths may besimilar but distinct. In other embodiments, first target beam 212 andsecond target beam 214 may be coupled by a target optical coupler 226into a combined target beam 252 prior to emission that may be directedtoward target 216 along a common optical path. In some embodiments,combined target beam 252 (or first target beam 212 and second targetbeam 214, if directed toward target 216 separately) may be reflected bytarget 216 and may be received by laser radar system 210 along areception optical path separate from the common optical path thatdirected combined target beam 252 toward target 216. Such embodimentsmay be labeled “bistatic.” Or, combined target beam 252 may be receivedby laser radar system 210 as a reflected target beam 256 along thecommon optical path. These latter embodiments may be labeled“monostatic.” Monostatic embodiments may provide advantages over theirbistatic counterparts when operating with reciprocal optics. Inmonostatic embodiments, the common optical path may include opticalmember 228 that may provide a common port for emitting combined targetbeam 252 and receiving reflected target beam 256. Optical member 228 mayinclude an optical circulator, an optical coupler or other opticalmember as would be apparent.

In some embodiments, the common optical path may include a scanningelement 257. Scanning element 257 may include an optical element suchas, for instance, a mirror, a lens, an antennae, or other opticalelements that may be oscillated, rotated, or otherwise actuated toenable combined target beam 252 to scan target 216. In some instances,scanning element 257 may enable scanning at high speeds. In conventionalsystems, scanning elements may be a source of mirror differentialDoppler noise effects due to speckle or other optical effects that maydegrade the accuracy of these systems. However, because variousembodiments of laser radar system 210 use simultaneous measurements (orsubstantially so) to unambiguously determine range and range rate,inaccuracies otherwise induced by high speed scanning may be avoided.

In some embodiments of the invention, a target optical coupler 254 maydivide reflected target beam 256 into a first reflected target beamportion 258 and a second reflected target beam portion 260. First localoscillator optical coupler 230 may combine first local oscillator beam242 with first reflected target beam portion 258 into a first combinedtarget beam 262. Second local oscillator optical coupler 232 may combinesecond local oscillator beam 248 with second reflected target beamportion 260 into a second combined target beam 264. In some embodimentsnot shown in the drawings, where, for example first target beam 212 andsecond target beam 214 may be directed to and/or received from target216 separately, first local oscillator optical coupler 230 may combinefirst target beam 212 that is reflected with first local oscillator beam242 to create first combined target beam 262, and second target beam 214that is reflected may be combined with second local oscillator beam 248to create second combined target beam 264.

Because first local oscillator beam 242 and second local oscillator beam248 may be combined with different target beams, or different portionsof the same target beam (e.g. reflected target beam 256), first combinedtarget beam 262 and second combined target beam 264 may representoptical signals that would be present in two separate, but coincident,single laser source frequency modulated laser radar systems, just priorto final processing. For example, laser source controller 236, firstlaser source 218, first optical coupler 222, first beam delay 244, andfirst local oscillator optical coupler 230 may be viewed as a firstlaser radar section 274 that may generate first combined target beam 262separate from second combined target beam 264 that may be generated by asecond laser radar section 276. Second laser radar section 276 mayinclude laser source controller 238, second laser source 220, secondoptical coupler 224, second beam delay 250, and second local oscillatoroptical coupler 232.

In some embodiments, laser radar system 210 may include a processor 234.Processor 234 may include a detection module 266, a mixing module 268, aprocessing module 270, and/or other modules. The modules may beimplemented in hardware (including optical and detection components),software, firmware, or a combination of hardware, software, and/orfirmware. Processor 234 may receive first combined target beam 262 andsecond combined target beam 264. Based on first combined target beam 262and second combined target beam 264, processor 234 may generate therange signal and the range rate signal. Based on the range signal andthe range rate signal, the range and the range rate of target 216 may beunambiguously determined.

In some embodiments of the invention, processor 234 may determine afirst beat frequency of first combined local oscillator beam 262. Thefirst beat frequency may include a difference in frequency, attributableto a difference in path length, of first local oscillator beam 242 andthe component of reflected target beam 256 that corresponds to firsttarget beam 212 that has been reflected from target 216. Processor 234may determine a second beat frequency of second combined localoscillator beam 264. The second beat frequency may include a differencein frequency, attributable to a difference in path length, of secondlocal oscillator beam 248 and the component of reflected target beam 256that corresponds to second target beam 214 that has been reflected fromtarget 216. The first beat frequency and the second beat frequency maybe determined simultaneously (or substantially so) to cancel noiseintroduced by environmental or other effects. One or more steps may betaken to enable the first beat frequency and the second beat frequencyto be distinguished from other frequency components within firstcombined target beam 262, other frequency components within secondcombined target beam 264, and/or each other. For example, these measuresmay include using two separate chirp rates as the first chirp rate andthe second chirp rate, delaying first local oscillator beam 242 andsecond local oscillator beam 250 for different delay times at first beamdelay 244 and second beam delay 250, respectively, or other measures maybe taken.

It will be appreciated that while FIG. 2 illustrates an exemplaryembodiment of the invention implemented primarily using optical fibersand optical couplers, this embodiment is in no way intended to belimiting. Alternate embodiments within the scope of the invention existin which other optical elements such as, for example, prisms, mirrors,half-mirrors, beam splitters, dichroic films, dichroic prisms, lenses,or other optical elements may be used to direct, combine, direct, focus,diffuse, amplify, or otherwise process electromagnetic radiation.

According to various embodiments of the invention, processor 234 may mixfirst combined target beam 262 and second combined target beam 264 toproduce a mixed signal. The mixed signal may include a beat frequencysum component that may correspond to the sum of the first beat frequencyand the second beat frequency, and a beat frequency difference componentthat may correspond to the difference between the first beat frequencyand the second beat frequency. For a target having constant velocity,first laser beam 240 and second laser beam 246 beat frequencies may bedescribed as follows:

$\begin{matrix}{{{f_{1}(t)} = {\frac{4\pi \; v}{\lambda_{1}} + {2{{\pi\gamma}_{1}\left( {R - {RO}_{1}} \right)}}}},{and}} & (1) \\{{{f_{2}(t)} = {\frac{4\pi \; v}{\lambda_{2}} + {2{{\pi\gamma}_{2}\left( {R - {RO}_{2}} \right)}}}},{respectively},} & (2)\end{matrix}$

where f₁(t) represents the first beat frequency, f₂(t) represents thesecond beat frequency, λ₁ and λ₂ are the two optical wavelengths, v isthe target velocity, γ₁ and γ₂ are proportional to the respective chirprates, R is the measured range and RO₁ and RO₂ represent the rangeoffsets for the two laser radars. Now assume that λ₁=λ₂=λ. We maysubtract the equations to yield

f ₁(t)−f ₂(t)=2πR(γ₁−γ₂)−2π(γ₁ RO ₁=γ₂ RO ₂)  (3)

Rearranging (3) we obtain

$\begin{matrix}{R = {\frac{\left( {{f_{1}(t)} - {f_{2}(t)}} \right)}{2{\pi \left( {\gamma_{1} - \gamma_{2}} \right)}} + \frac{\left( {{\gamma_{1}{RO}_{1}} - {\gamma_{2}{RO}_{2}}} \right)}{\left( {\gamma_{1} - \gamma_{2}} \right)}}} & (4)\end{matrix}$

as the corrected range measurement. Similarly we may combine (1) and (2)to obtain the expression,

$\begin{matrix}{{v = {{\frac{\lambda}{4\pi}\left( \frac{{f_{1}(t)} - {\frac{\gamma_{1}}{\gamma_{2}}{f_{2}(t)}}}{1 - \frac{\gamma_{1}}{\gamma_{2}}} \right)} + {\frac{{\lambda\gamma}_{1}}{2}\left( \frac{{RO}_{1} - {RO}_{2}}{1 - \frac{\gamma_{1}}{\gamma_{2}}} \right)}}},} & (5)\end{matrix}$

which provides a measure of the target velocity.

According to various embodiments of the invention, the beat frequencysum component, described above in Equation 4, may be filtered from themixed signal to produce a range signal. From the beat frequency sumcomponent included in the range signal (e.g. f1(t)+f2(t)), adetermination of the distance from laser radar system 210 to target 216may be made. The determination based on the range signal may beunambiguous, and may not depend on either the instantaneous behavior, orthe average behavior of the Doppler frequency shift (e.g. v/λ).

In some embodiments, the beat frequency difference component, describedabove in Equation 4, may be filtered from the mixed signal to produce arange rate signal. From the beat frequency difference component includedin the range rate signal (e.g. Error! Objects cannot be created fromediting field codes.) a determination of the range rate of target 216may be unambiguously made. To determine the range rate of target 216,

${f_{1}(t)} - {\frac{\gamma_{1}}{\gamma_{2}}{f_{2}(t)}}$

may be represented as a value proportional to a chirp rate differencebetween the first chirp rate and the second chirp rate. This may enablethe Doppler shift information to be extracted, which may represent aninstantaneous velocity of target 216.

In some embodiments of the invention, the second chirp rate may be setto zero. In other words, second laser source 218 may emit radiation at aconstant frequency. This may enable second laser source 218 to beimplemented with a simpler design, a small footprint, a lighter weight,a decreased cost, or other enhancements that may provide advantages tothe overall system. In such embodiments, laser radar system 210 mayinclude a frequency shifting device. The frequency shifting device mayinclude an acousto-optical modulator 272, or other device.Acousto-optical modulator 272 may provide a frequency offset to secondlocal oscillator beam 248, which may enhance downstream processing. Forexample, the frequency offset may enable a stationary target beatfrequency between second local oscillator beam 248 and second reflectedtarget beam portion 260 representative of a range rate of a stationarytarget to be offset from zero so that the a direction of the target'smovement, as well as a magnitude of the rate of the movement, may bedetermined from the beat frequency. This embodiment of the invention hasthe further advantage that it may allow for continuous monitoring of thetarget range rate, uninterrupted by chirp turn-around or fly-back. Chirpturn-around or fly-back may create time intervals during which accuratemeasurements may be impossible for a chirped laser radar section. Inthese embodiments, laser radar section 276 may only determine the rangerate of target 216 while laser radar system 210 retains the ability tomeasure both range and range rate.

FIG. 4 illustrates a processor 234 according to one embodiment of theinvention. Processor 234 may mix first combined target beam 262 andsecond combined target beam 264 digitally. For example, processor 234may include a first detector 410 and a second detector 412. The firstdetector 410 may receive first combined target beam 262 and may generatea first analog signal that corresponds to first combined target beam262. The first analog signal may be converted to a first digital signalby a first converter 414. Processor 234 may include a first frequencydata module 416 that may determine a first set of frequency data thatcorresponds to one or more frequency components of the first digitalsignal. In some instances, the first digital signal may be averaged at afirst averager module 418. In such instances, the averaged first digitalsignal may then be transmitted to first frequency data module 416.

Second detector 412 may receive second combined target beam 264 and maygenerate a second analog signal that corresponds to second combinedtarget beam 264. The second analog signal may be converted to a seconddigital signal by a second converter 420. Processor 234 may include asecond frequency data module 422 that may determine a second set offrequency data that corresponds to one or more of frequency componentsof the second digital signal. In some instances, the second digitalsignal may be averaged at a second averager module 424. In suchinstances, the averaged second digital signal may then be transmitted tosecond frequency data module 422.

The first set of frequency data and the second set of frequency data maybe received by a frequency data combination module 426. Frequency datacombination module 426 may linearly combine the first set of frequencydata and the second set of frequency data, and may generate a range ratesignal and a range signal derived from the mixed frequency data.

FIG. 5 illustrates a processor 234 according to another embodiment ofthe invention. Processor 234 may include a first detector 510 and asecond detector 512 that may receive first combined target beam 262 andsecond combined target beam 264, respectively. First detector 510 andsecond detector 512 may generate a first analog signal and a secondanalog signal associated with first combined target beam 262 and secondcombined target beam 264, respectively. Processor 234 may mix firstcombined target beam 262 and second combined target beam 264electronically to generate the range signal and the range rate signal.For example, processor 234 may include a modulator 514. Modulator 514may multiply the first analog signal generated by first detector 510 andthe second analog signal generated by second detector 512 to create acombined analog signal. In such embodiments, processor 234 may include afirst filter 516 and a second filter 518 that receive the combinedanalog signal. First filter 516 may filter the combined analog signal togenerate a first filtered signal. In some instances, first filter 516may include a low-pass filter. The first filtered signal may beconverted by a first converter 520 to generate the range rate signal.Second filter 518 may filter the combined analog signal to generate asecond filtered signal. For instance, second filter 518 may include ahigh-pass filter. The second filtered signal may be converted by asecond converter 522 to generate the range signal.

FIG. 6 illustrates a processor 234 according to yet another embodimentof the invention. Processor 234 may mix first combined target beam 262and second combined target beam 264 optically to generate the rangesignal and the range rate signal. For example, processor 234 may includea detector 610 that receives first combined target beam 262 and secondcombined target beam 264 and generates a combined analog signal based onthe detection. In such embodiments, processor 234 may include a firstfilter 612 and a second filter 614 that receive the combined analogsignal. First filter 612 may filter the combined analog signal togenerate a first filtered signal. First filter 612 may include alow-pass filter. The first filtered signal may be converted by a firstconverter 616 to generate the range rate signal. Second filter 614 mayfilter the combined analog signal to generate a second filtered signal.Second filter 14 may include a high-pass filter. The second filteredsignal may be converted by a second converter 618 to generate the rangesignal.

While the invention has been described herein in terms of variousembodiments, it is not so limited and is limited only by the scope ofthe following claims, as would be apparent to one skilled in the art.

1. A laser radar system comprising: a first coherent laser radar sectionincluding: a laser source that generates a first laser beam, aninterferometer that splits the first laser beam into a first target beamand a first local oscillator beam, and generates a first combined targetbeam from a first reflected portion of the first target beam and thefirst local oscillator beam; a second coherent laser radar sectionincluding: a laser source that generates a second laser beam, aninterferometer that splits the second laser beam into a second targetbeam, incident on the target at the same location as the first targetbeam, and a second local oscillator beam, and generates a secondcombined target beam from a second reflected portion of the secondtarget beam and the second local oscillator beam; and a processor thatdetermines a range and a range rate of the target unambiguously from thefirst combined target beam and the second combined target beam.
 2. Thesystem of claim 1, wherein the first chirp rate is in the oppositedirection from the second chirp rate.
 3. The system of claim 1, furthercomprising: a combiner that generates a combined target beam from thefirst target beam and the second target beam prior to emission; and aninput/output port that directs the combined target beam toward thetarget and receives reflected electromagnetic radiation reflected fromthe target to form a portion of a common optical path.
 4. The system ofclaim 3, wherein the input/output port is provided by one or more of acirculator, an optical coupler, and an optical fiber.
 5. The system ofclaim 3, wherein the common optical path includes a scanning elementthat enables the first target beam and the second target beam to bescanned across the target.
 6. The system of claim 3, wherein the firstlaser source and the second laser source have substantially the sameoptical frequency.
 7. The system of claim 1, wherein the processorelectrically mixes a first beat signal related to the first combinedtarget beam and a second beat signal related to the second combinedtarget beam.
 8. The system of claim 1, wherein the first localoscillator beam is divided into a plurality of first local oscillatorbeams and the second local oscillator beam is divided into a pluralityof local oscillator beams, and the plurality of first local oscillatorbeams and the plurality of second local oscillator beams are delayed forvarying delay times to ensure that the range of the target will fallwithin a set of ranges for which the determination of the range and therange rate by the system will be accurate.
 9. The system of claim 1,wherein at least one of the first laser source and the second lasersource comprise: one or more optical elements that form a optical cavitythrough which electromagnetic radiation is circulated; and a frequencyshifting device that applies a frequency shift to the electromagneticradiation circulating through the optical cavity.
 10. The system ofclaim 9, wherein the frequency shifting device applies a constantfrequency shift to the electromagnetic radiation circulating through theoptical cavity.
 11. The system of claim 9, wherein the frequencyshifting device is an acousto-optic Bragg cell.
 12. The system of claim1, wherein at least one of the first laser source and the second lasersource is periodically and automatically linearized.
 13. The system ofclaim 12, wherein the at least one of the first laser source and thesecond laser source is automatically linearized once per chirp.
 14. Thesystem of claim 1, wherein the first target beam and the second targetbeam are focused on the common measurement point.
 15. A laser radarsystem comprising: a first coherent laser radar section including: alaser source that generates a first laser beam at a first chirp rate, aninterferometer that splits the first laser beam into a first target beamand a first local oscillator beam, and generates a first combined targetbeam from a first reflected portion of the first target beam and thefirst local oscillator beam; a second coherent laser radar sectionincluding: a laser source that generates a second laser beam at a fixedfrequency, an interferometer that splits the second laser beam into asecond target beam, incident on the target at the same location as thefirst target beam, and a second local oscillator beam, and generates asecond combined target beam from a second reflected portion of thesecond target beam and the second local oscillator beam; and a processorthat determines a range and a range rate of the target unambiguouslyfrom the first combined target beam and the second combined target beam.16. The system of claim 15, wherein the second laser radar sectionfurther includes a frequency shifting device that provides a frequencyshift to the second local oscillator beam.
 17. The system of claim 15,further comprising: a combiner that generates a combined target beamfrom the first target beam and the second target beam prior to emission;and an input/output port that directs the combined target beam towardthe target and receives reflected electromagnetic radiation reflectedfrom the target to form a portion of a common optical path.
 18. Thesystem of claim 17, wherein the input/output port is provided by one ormore of a circulator, an optical coupler, and an optical fiber.
 19. Thesystem of claim 17, further comprising a scanning element, included inthe combined optical path, that receives the combined target beam fromthe input/output port and scans the first target beam and the secondtarget beam across the target.
 20. The system of claim 17, wherein thefirst laser source and the second laser source have substantially thesame optical frequency.
 21. The system of claim 15, wherein the firstlocal oscillator beam is divided into a plurality of first localoscillator beams and the second local oscillator beam is divided into aplurality of local oscillator beams, and the plurality of first localoscillator beams and the plurality of second local oscillator beams aredelayed for varying delay times to ensure that the range of the targetwill fall within a set of ranges for which the determination of therange and the range rate by the system will be accurate.
 22. The systemof claim 15, wherein the first laser source comprises: one or moreoptical elements that form a optical cavity through whichelectromagnetic radiation is circulated; and a frequency shifting devicethat applies a frequency shift to the electromagnetic radiationcirculating through the optical cavity.
 23. The system of claim 22,wherein the frequency shifting device applies a constant frequency shiftto the electromagnetic radiation circulating through the optical cavity.24. The system of claim 22, wherein the frequency shifting device is anacousto-optic Bragg cell.
 25. The system of claim 15, wherein the firstlaser source is automatically and periodically linearized.
 26. Thesystem of claim 25, wherein the first laser source is periodicallylinearized once per chirp.
 27. The system of claim 15, wherein the firsttarget beam and the second target beam are focused on the commonmeasurement point.